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The early postnatal development of neurons containing vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) has been analyzed in visual areas 17 and 18 of cats aged from postnatal day (P) 0 to adulthood. Neuronal types are established mainly by axonal criteria. Both peptides occur in the same neuronal types and display the same postnatal chronology of appearance. Several cell types are transient, which means that they are present in the cortex only for a limited period of development. According to their chronology of appearance the VIP/PHI-immunoreactive (ir) cell types are grouped into three neuronal populations.
The first population comprises six cell types which appear early in postnatal life. The pseudohorsetail cells of layer I possess a vertically descending axon which initially gives rise to recurrent collaterals, then forms a bundle passing layers III to V, and finally, horizontal terminal fibers in layer VI. The neurons differentiate at P 4 and disappear by degeneration around P 30. The neurons with columnar dendritic fields of layers IV/V are characterized by a vertical arrangement of long dendrites ascending or descending parallel to each other, thus forming an up to 600 μm long dendritic column. Their axons always descend and terminate in broad fields in layer VI. The neurons appear at P 7 and are present until P 20. The multipolar neurons of layer VI occur in isolated positions and have broad axonal territories. The neurons differentiate at P 7 and persist into adulthood. Bitufted to multipolar neurons of layers II/III have axons descending as a single fiber to layer VI, where they terminate. The neurons appear at P 12 and persist into adulthood. The four cell types described above issue a vertically oriented fiber architecture in layers II-V and a horizontal terminal plexus in layer VI which is dense during the second, third and fourth week. Concurrent with the disappearance of the two transient types the number of descending axonal bundles and the density of the layer VI plexus is reduced, but the latter is maintained during adulthood by the two persisting cell types.
Two further cell types belong to the first population: The transient bipolar cells of layers IV, V, and VI have long dendrites which extend through the entire cortical width. Their axons always descend, leave the gray matter, and apparently terminate in the upper white matter. The neurons differentiate concurrently with the pseudohorsetail cells at P 4, are very frequent during the following weeks, and eventually disappear at P 30. The persisting bipolar cells of layers II/III and IV have shorter dendrites and issue a diffuse axon plexus which largely remains intralaminarly. The neurons are recognizable from P 12 and persist, although in the adult cat cortex they are rare.
The second population comprises two cell types. The neurons with uertically descending main axon and horizontal collaterals of layers II/III and IV form diffuse axon plexuses by means of horizontal and oblique collaterals. They seem to terminate in the form of en passant boutons on pyramidal cell bodies. The neurons appear a t P 18 and persist into adulthood. The neurons of layer I have broad dendritic fields and unconspicuous short axons which diffusely distribute in superficial layers. The cells are recognizable from P 25 onwards. The appearance of the second population initiates a change from the so far vertically organized architecture to a more diffuse innervation pattern. The change is complete when the transient innervation patterns have become superseded during the second postnatal month by the basket cells of layers II/III and IV, which constitute the third population. These neurons by far outnumber all other cell types and, until the end of the second month, fill these layers with a dense terminal plexus. Thereafter, the number of somata and the terminal density is reduced in layer IV, so that in adult animals most neurons and the densest plexus reside in layers II/III. The innervation now is less dense in layer IV and only loose in deeper cortical layers.
In summary, transient VIP- and PHI-ir cell types dominate the early postnatal cortex. They form a largely vertical fiber architecture, and degener- ate and become eliminated by probably cell death around the end of the first month. Concurrently with their disappearance and during the second postnatal month, the persisting cell types form a completely different innervation pattern which reaches an adult state late postnatally.
